Tanks are used to store a wide variety of fluids and gasses, especially petroleum, as well as other liquids containing or consisting of various chemicals, and also for storing water. Many of these tanks and/or piping connected to the tanks when tested have been found to have active leaks or the potential for leaks. Some current estimates indicate that from 1% to 45%, or even more of such tanks have leaks. Leaking tanks and leaking tank piping can contaminate the ground water and cause other types of environmental pollution which become health and safety hazards.
In recent years leak detection methods have been developed to test underground storage tanks for leaks. In December 1988, the United States Environmental Protection Agency (U.S. EPA) adopted regulations requiring that such leak detection methods for underground storage tanks meet certain leak detection criteria, i.e. the ability to detect a 0.1 gph (gallon per hour) leak with a probability of detection of 95% and a false alarm rate of 5%. No known leak detection method has been able to meet this U.S. EPA leak detection requirement for (1) an empty tank and associated piping or (2) tank and piping together in which the tank is partially filled with liquid to less than 70% of its volume.
Several attempts have been made to solve this problem (i.e. U.S. Pat. Nos. 4,568,925; 4,353,245; 3,918,291; 3,893,332; 4,791,814; and 4,852,054). Each of these prior art devices and/or methods have certain disadvantages which make them unacceptable for detecting leakage in underground storage tanks of the low pressure variety. The testing methods in several of the cited patents require a precise measurement of the liquid level in the tank. These methods are unable to detect tank leaks where the leak is located above the liquid level in the tank or in its associated piping. Other methods require that the tank be pressurized to a very high pressure (i.e. 4000 psi for submarines, in order to determine the acceptability of leakage). Still other methods require that a nitrogen or helium detector be employed in such a way as to detect leakage of such gasses which are used to pressurize the tank. These methods require that the tank leakage flows be directed to the gas leak detector or that detector access to all parts of the tank and piping be provided so that the leak detector apparatus can sense low gas concentrations (indicating small leaks) as required by the EPA standards.
Prior art methods have been somewhat successful for detecting leaks in generally full tanks and tanks which are able to sustain high internal pressures. However, prior art methods have not been successful in meeting the EPA guidelines for empty or partially filled generally low pressure tanks and associated piping. The test results from prior art methods and devices are influenced by many of the effects associated with the underground and aboveground storage tanks such as: tank deflection, temperature changes, ground water, tank structure, ullage space, product level, and product type. The interaction of these effects can and does cause unacceptably inconsistent results from the prior art test devices and methods.